We are investigating two viruses of the Toga virus group -- Sindbis and the lactic dehydrogenase (LD) virus. The specific studies with Sindbis virus include: (1) the formation and function of viral glycoproteins, and (2) a characterization of defective viral particles. Continued passage of Sindbis virus in BHK cells gives rise to defective particles that interfere with the replication of the normal virion. The initial experiments with the LD virus will be devoted to the development of assays to detect the virus and to the recognition of infected cells in culture. Studies with defective particles from Sindbis and with LD virus will serve as models for investigating slowly progressing viral diseases and persistent viral infections.